A specter haunts the Western powers: biological terrorism. The idea of the “microbial storm“ betrays a considerable amplification in scope for health security. How did this worst-case-scenario logic start? What is actually at stake when it is applied to defense against bacterial threats? Patrick Zylberman clarifies three major areas: the heuristic role of fiction in the conceiving and testing of crisis management procedures; worst-case logic as a rational system of dealing with microbial crises; and the organization of public service bodies.